FIG. 1 illustrates a related art video decoding method in an Reference Picture Resampling (RPR) mode. It is noted that the RPR mode, utilized in video standards such as H.263+ and RealVideo standards, is a mode allows video encoder or decoder to change picture size on a frame-by-frame basis. As shown, a bit stream is decoded to generate a plurality of frames, and different classes of frames are respectively stored to a plurality of buffers, where the first and second sequences respectively show the frame decoding order and corresponding storing buffers. For example, an I frame I0 is stored in the reference buffer RB1, a P frame P1 is stored in the reference buffer RB2, B frames B2 and B3 are stored in the B frame buffer BB. When decoding frames encoded in the RPR mode; however, it is required to scale a corresponding reference frame and an additional RPR frame buffer is required to store scaled frames such as P frames P1s and P4L which respectively represent the scaled picture of P frame P1 with smaller resolution and the scaled picture of P frame P4 with larger resolution. The necessity of the RPR frame buffer increases the related cost.